Cell, Protein, Cell division, Research, Cell cycle Lecture 11 Mitosis and Meiosis
This semester is going pretty fast, even though we are online and never see each other in person. Our third set of material deals with reproduction we’re, going to start today with lecture number 11, which is an overview of the processes of mitosis and meiosis it’s important that you really know the outcome of those two types of cellular division and know what cells undergo mitosis And meiosis before we begin actually talking about male and female reproductive systems. So for today, if you are an expert on meiosis and mitosis, if you understand its principles, you can pretty much just read through this hand down and skip right over it. But if you need some help that’s, what today is designed for so i’m going to screen share with you today our notes that are in chapter 11 or lecture 11. I should say all about mitosis and meiosis. Now our cells undergo a life cycle that is similar to that of our whole organismal life cycle. Our cells have a period of time in which they just do. Their thing may make some product that they excrete they may contract. They may send impulses. All of that kind of stuff, and then those cells undergo a process of cellular division. Cellular division occurs when we copy the dna that’s in the nuclei of these cells and then divide that dna, so that each new cell gets an adequate amount of that genetic material. The process of doing such is known as mitosis mitosis results in two identical offspring cells.
We call those daughter cells. These cells, if everything happens, normally, are genetically identical to that of the parent cell. Now mitosis takes place in the microbial world as well as in us. Mitosis in the microbial world is what enables us to grow from one cell, a whole colony of cells that you can actually see on a petri dish in multicellular. Organum organisms like us, mitosis equates to growth. Mitosis happens in all of our body cells, but it happens at different rates and in some cells it stops during our lifespan. For example, we talked about our neurons. Our neurons are thought to be not mitotically active when our nervous system is mature. That means that, as our brain develops and our spinal cord develops before birth, we have mitosis taking place in those cells, but when those structures reach their maturity, no more cells are formed. If you contrast that, however, to cells like skin cells, our skin cells continually reproduce, we shed this outer layer of skin as new skin cells form from the basal layer and are pushed upwards. So some cells lines in our body have mitotic cycles that are lengthy and last our whole lifetime. Others are only active at specific times in the nucleus of our cells. We find dna. The dna is long strands of nucleotides that represent the genetic instructions for our cells. To manufacture proteins, now, most of our cell cycle time, this dna is spent wrapped around histone proteins into a nice dense packet at the time cell division nears, however, the dna expands and then contracts makes a copy of itself, which is known as dna replication and our Chromosomes are formed i’m going to scroll down here to this image here i want you to see a pair of chromosomes.
If we look at this side, this structure is one set of dna and that dna set. We call a chromatid this one over here is a similar set of dna, but you can tell that they are not identical for simplicity’s sake. Let’S pretend that these two chromosomes, the yellow one and the orange one, are responsible for giving us a certain eye color. Maybe this yellow one says: we’ll have blue eyes and this red one or brown. One says that we’ll have brown eyes. Both of them have genetic information concerning eye color, but the precise information is a little different in each one. When our dna replicates. What we do is we make a copy of the yellow one and a copy of the brown one, and we keep those copies stuck together by a sticky area known as the centromere. So after replication we have one chromosome attached to an identical chromosome. At a sticky point known as the centromere here’s ours for brown eyes, we have one chromatid stuck to another chromatid and the sticky part in the middle is the centromere. These two chromosomes both tell us how to make eye color. This one says: make blue eye color. This one says make that individual have brown eyes, so the exact genetic information between yellow and brown is different, but it’s about the same trait. However, within the chromosome, this side of our chromosome and this side of our chromosome is identical same thing here this side and this side are identical.
We call these two identical pieces of dna sister chromatids, the term homologous, chromosomes or homologous chromosomes here means we control the same set of traits, but there may be some genetic variety that’s there. Now in humans we have a total of 46 chromosomes in pairs. Each pair is alike in size and shape and function. That means they’re, homologous chromosomes and because our chromosomes are in pairs. We say that these these are diploids. Diploid can be designated as 2n, where n indicates the number of different types of chromosomes present. These two images show you a karyotype one of the things i used to do before i started teaching was, i worked for the state genetics lab and we did a lot of karyotyping what you do for karyotyping. Is you take some cells? They could be cells in the amniotic fluid of a child before it’s born or they could be just simply cheek cells. You grow those cells up in a petri dish. You harvest those cells and expose them to a specific chemical that stops dna movement. It it freezes. The cells at that particular level of their activity, and then you make a picture of that when you get that picture made you blow it up, so we’ve got a big image now and you cut out every little chromosome and, as you can see in this photograph Here some of our chromosomes have light patterns and dark patterns. They have different shapes. This one has a small top, but a big bottom.
This one has looks like dots and that are very small and then a different banding at the bottom. You plot those on this special kind of paper that numbers these, so that you can find our 23 chromosomes. We have 22 chromosomes that are alike in males and females, and then we have six chromosomes. Our sex chromosomes are either two x’s like we see here and that individual’s a female or we have an x and a small chromosome and that’s the y one, and that gives that person male characteristics, so a chromosomes can be, can be um graphically represented in this Karyotype, these cells are diploid 23 pairs of chromosomes. Everything impairs now. If we produced cells through mitosis all the time, then we would start with our off our parent cell. We would undergo dna replication. We would undergo cell division and we’d end up with two identical daughter cells. Look how these two have a blue and a green just like the mother cell did or the parent cell. This one has a blue and a green, just like the parent cell. So if we underwent mitosis with this scroll down a little bit with this individual here, we would end up with two cells that have 46 chromosomes they’re diploid in pairs. Mitosis is a great way to keep the the consistency in generation from generation. We want all of our skin cells that form to be genetically identical to the parent cell. That way they all function in an identical way, but it does not work for sexual reproduction.
Mitosis is a type of asexual reproduction in sexual reproduction. We have to have a union of the sperm and the egg so that we get part of our dna material from mom part of our dna material from dad. If we had sexual reproduction involving cells that underwent mitosis, we would have 46 chromosomes donated from dad and 46 chromosomes donated to that baby by mom. The baby would have 92 chromosomes and then maybe when that baby grew up 40 92 chromosomes from that baby and 92. With its partner, the next generation, the grandchildren, if you will, would have 184 chromosomes, i think you can see how mitosis does not work for sexual reproduction in sexual reproduction. We use a process known as meiosis. Meiosis only occurs in cells that develop into sperm cells or egg cells. Nothing else. Every other cell in your body develops either through mitosis or undergoes mitosis, to reproduce itself only cells that become sperm or egg undergo meiosis. Now meiosis is a two step process. It’S a process very similar to mitosis, but what it does for us is it separates the chromosome pairs. This diagram shows you meiosis one and the second phase meiosis ii in meiosis, one we have our four chromosomes. These four chromosomes or the two pairs of chromosomes are separated so that by the time meiosis one is over, each developing cell has one member of each chromosome pair. Let me say that again by the time meiosis one is over our 46 chromosomes.
Our pairs of chromosomes are separated. Each new cell has 23 chromosomes one member of each pair meiosis ii occurs by separating the chromatids it’s, actually identical to what takes place in mitosis, the chromosomes align in the center of the cell. We pull the chromatids apart. We end up now with four cells each with a complete set of half of the dna, so the dna in this blue one and red one is identical to the dna. Here we haven’t separated chromatids we’ve, separated copies of that dna. As a result, meiosis lets us change our chromosomal number from the diploid 46 or 23 pairs to a haploid 23 chromosomes with no pairs. We designate haploid as a little n, so the takeaway from today’s lesson is as follows: mitosis takes place in all growing body tissues following mitosis. The daughter cells are genetically identical to the parent cell for humans. We had 23 pairs of chromosomes diploid cells. We end up with daughter cells that also have 23 pairs. Diploid 46 total chromosomes meiosis occurs only in cells that become sperm cells or egg cells. It involves two rounds of cell division in the first round of cell division called meiosis 1. We separate the pairs so that each daughter cells has 23 chromosomes no pairs and is considered to be haploid. Meiosis 2 is a cell division. That’S identical to mitosis, chromatids are separated and the centromeres dissolve to to allow that separation to occur. The resulting cells are haploid.
We’Re going to take these concepts of meiosis and mitosis and apply them to the male and female reproductive systems. We’Re going to start with the male reproductive system, because, to be honest with you, it’s a little more simplistic than what goes on in the female, and i think you’ll see why, when we talk about the females, responsibility and function as these cells are produced, so i Hope this makes sense. If you have questions about mitosis and meiosis, please let me know it’s really important, that you get this understanding of what’s going on with it. You don’t have to memorize all these little phases like prophase, metaphase, anaphase and telophase. If you just know this take away part here and have a really clear understanding of that, you should be in good good stead for our upcoming lectures, as always there’s some questions for you to answer see if you can go through these see. If you can answer those to get you ready for the material on our third exam that will be here before you know it. If you have questions about anything, please don’t hesitate to.